Hey bloke of always tried to have about 30 odd% on tow ball but i've had 4wd's for the last 15yr's so you don't really notice it that much.I've built a few trailers for mates and my own and we've pretty much made a copy of an existing trailer and have never had any issue's with towing maybe the spring's are a bit heavy like you say but it's a bit of a stuff around to change them on a guess try stickin a bit extra weight in it and try it out. Cheers Dan......